The biology and pathology of natural and induced tumors are characterized. Natural tumors are characteristic for each strain of rat, while induced tumors may be different biologically and morphologically from natural tumors. Natural tumors from approximately 150 F344 rats were transplanted into the mammary fat pad of weanling F344 rats. The tumor latent period, growth rate, invasiveness and metastatic rate were directly related to the histologic appearance of the tumor and tumor cell type. Both malignant and benign tumors were transplanted although only the former were invasive or metastatic in syngeneic hosts. The experiment provides evidence that characteristics of tumor transplantation can be indicative of the degree of malignancy of a primary tumor, but not whether it is benign. Leukemia in the F344 rat was identified as a large granular lymphocyte leukemia, the major effective cell for natural killer activity in the rat. Its role in natural tumor immunity is being studied.